Influence of postnatal rearing conditions on the response of squirrel monkey infants to brief perturbations in mother-infant relationships

Abstract

This study was designed to determine the behavioral and pituitary-adrenal responses of squirrel monkey mother-infant dyads reared under different housing conditions to either 1- or 6-hr separations. Dyads were reared either in an individual cage or in social groups of 3 mother-infant dyads. The two separation conditions consisted of removing either the mother or the infant to a novel test cage while the other member remained in the home cage. Group-reared infants displayed lower levels of plasma cortisol, movement, and vocalization when they remained in the home cage during separation compared to their responses in the novel cage. However, individually-reared infants displayed high cortisol and activity levels in both separation environments, and vocalization levels were higher at 1 hr in the home cage than in the novel cage. These results indicate that familiarity with the separation environment, per se, does not attenuate the behavioral or physiological responses of infants, but that familiar conspecifics, even in the absence of alloparenting, can benefit an infant during separation from its mother. Two additional test conditions assessed the responses of mother-infant dyads when they were only momentarily separated and then immediately reunited in either the home cage or a novel cage. Reunion in the home cage evoked no cortisol or behavioral responses, but reunion in a novel cage resulted in significant elevations in infant cortisol levels and time in contact with the mother. The corticoid response of the mothers differed from their infants during separations. Although when placed alone in a novel cage, all mothers had elevated cortisol levels after 6 hr, only group-reared mothers allowed to remain at home displayed cortisol elevations after 1 hr of separation. Mother in both rearing conditions had elevated corticoid levels following separation/reunion in the novel cage at 1 hr but showed no further elevation at 6 hr. These results indicate that familiar environments, even in the absence of social partners, buffer the mother's response to separation.